Long-term exposure to high levels of noise may cause hearing loss and other health hazards, and as a consequence U.S. law requires that the noise exposure level of an individual in a work place be accurately measured. See e.g. 29 C.F.R. §1910.95, and U.S. Pat. No. 6,456,199 for a general discussion. A noise dosimeter with the microphone located near the workers' shoulder is commonly used in the industry to measure the individual cumulative noise exposure over the course of a full work shift. Since the microphone is located on the shoulder, the measurements do not account for any noise reduction afforded by hearing protectors and therefore the measurement quantity is unprotected individual exposure.
Hearing protectors are commonly used in industry to reduce the level of noise incident on the ear of the worker. U.S. Pat. No. 6,456,199, incorporated herein by reference, discloses a cost-effective and unobtrusive means of continuously monitoring a hearing protector wearer's actual, protected, noise exposure rather than simply measuring unprotected individual exposure.
The monitoring system includes at least one microphone, housed in the interior of a hearing protective device. Exposure dosage calculation includes periods when the HPD is worn (primary microphone position) and periods when it is not worn (secondary microphone position). When the HPD is worn at the primary position, it measures the noise level with the protective device in effect, and when the HPD is worn at the secondary position, it measures the noise level of the environment without the protective device. This provides an accurate measurement of the actual cumulative exposure dosage because invariably workers have their hearing protectors donned for part of the day and removed for part of the day.